A focused study is proposed here on the protein and glycoprotein components of a model mammalian cell membrane, the mouse L cell plasma membrane. These membranes will be prepared in three different ways and characterized morphologically by electron microscopy and biochemically by determining their relative content of protein, phospholipid and cholesterol. The protein and glycoprotein components associated with the three membrane preparations will be identified by their electrophoresis mobilities on SDS-polyacrylamide gels. Further characterization of individual membrane proteins will involve preparing peptide "fingerprints" and comparing them with similar fingerprints of known L cell proteins such as actin, myosin and tropomyosin. Individual membrane proteins will also be distinguished according to whether and under what circumstances they can become phosphorylated in vivo and in vitro. More detailed studies will be carried out on one particular L cell surface glycoprotein, a 100,000 MW Con A receptor called RHO. Structural studies in purified rho will be undertaken to determine the relative content of carbohydrate and polypeptide in the overall molecule. Also, antiserum directed against RHO will be employed to identify cross-reacting antigens recently found to be present on the surface of mouse T lymphocytes. The antiserum will also be employed to test directly whether RHO may be involved in the transport of nutrients into L cells.